Chemical method of coating aluminum



Patented Mar. 15,1949 f Ralph B. Mason, New Kensington, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa.', a corporation of Pennsylvania No Drawing. Application February 15, 1945, Serial No. 578,134

Claims. l

Aluminum surfaces, by which term the surfaces of aluminum and aluminum base alloys containing about '70 per cent or more by weight of that metal are herein collectively referred to, may by means of various processes be provided with an artificially produced coating which consists, in substantial part, of aluminum oxide. The process by which such a coating is produced may result in a coating which contains other components which may or may not alter specific characteristics of the coating. In the art such coatings are referred to as oxide coatings and, in various degrees depending upon the particular coating, they exhibit as general properties a thickness greater than the natural film of oxide which appears on all aluminum surfaces, some resistance to corrosion and abrasion, an ability to respond to treatment with organic dyes or inorganic coloring agents to form colored surfaces, permean bility and a capacity to adsorb or absorb moisture or liquids. By various treatments imposed after the oxide coatings have been formed, they may be adapted to widely varyin specific purposes, or the natural qualities and properties of the coating may be improved, modified or enhanced.

These useful oxide coatings may be produced on aluminum surfaces by a large choice of methods, some of which are superior to others, but such methods, regardless of their general desirability, usually fall in one of two classes. In one class of methods'the aluminum surface is made anode in an electrolytic cell, the electrolyte of which is a selected coating forming solution, and electrical current is passed to cause, or help cause, the formation of the coating on the aluminum surface. In the other class of methods the oxide coating is formed on the aluminum surface by chemical reaction and without the application of external electrical energy. In this case the aluminum surface is immersed in, or otherwise con-.

tacted by, the coating forming solution.

These chemical, or non elec'trolytic, methods of forming oxide coatings are usually less expensive in operation and do not require the use of the special implements required in connection with the production of electrolytic oxide coatings. However, the coating produced by the chemical treatment is usually not as hard, adherent and compact as those produced by properly operated electrolytic treatments l Despite the well known disadvantages of these chemical oxide coatings,

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they have found widespread application in the I art in many instances where, because of size, shape and relative cost of the product to be coated. the existing facilities of manufacture, the

amount of material to be coated'at a given time and similar or related factors, the electrolytic type of coating cannot be economically used. There are likewise instances where the relative disadvantages of chemically formed oxide coatings are not of controlling importance, as where the coated aluminum surface is to be put to uses which do not place severe demands upon the coating.

Of the aqueous solutions used to chemically form such oxide coatings on aluminum surfaces, those containing an alkali metal carbonate as the principal active constituent have proved to be the most useful in the commercial arts. These solutions usually contain about 0.5 to 6 per cent by weight of alkali metal carbonate, preferably sodium carbonate. In the use of such solutions it is sometimes preferred, depending upon the ultimate characteristics desired in the coating, to provide in the coating forming solution a quantity of soluble chromium, i. e. hexavalent chromium, best present as alkali metal chromate, a term which is used herein to include and describe the equivalent alkali metal dichromates.

I have found that the nature of oxide coatings chemically formed on aluminum surfaces by the use of these alkali metal carbonate solutions can be considerably improved if there is present in the coating forming solution an amount of polyvinyl alcohol. The addition of the polyvinyl alcohol to the solution promotes the formation of an oxide coating which is more adherent and more resistant to abrasion than the coating usually formed by the use of the alkali carbonate solution without the presence therein of polyvinyl alcohol. The amount of polyvinyl alcohol present in the coating forming solution may vary' from about 0.2 to about 5 per cent by weight of the total solution, but I prefer to use amounts of about 0.2 to 3 per cent by weight. In forming the coating, the metal surface to be treated is merely immersed in the solution, which may be agitated if desired, and is left therein for a period of time which will vary depending upon the nature of the aluminum surface being treated and the concentration of the solution. Generally speaking, good coatings may be produced in about 30 minutes if the temperature range of the solution is between about and C I have further determined that when the concentration of the alkali" metal carbonate and the polyvinyl alcohol are substantially equal, as expressed in per cent by weight of the total solution, the best coatings are usually obtained, although this particular rule does not hold as to all surfaces treated. I have obtained an aluminum alloy, the principal alloying con-:

stituent of which was about 4% per cent copper, I have observed that a tough, smooth, glossy coating was-best obtained when the treating solution contained about 3 per cent by weight of polyvinyl alcohol and 3 per cent by weight of sodium carbonate. Using a coating solution containing 1 per cent by weight of sodium carbonate.'1 per centlby weight of polyvinyl alcohol and 0.1 per cent by weight of sodium chromate, I have produced very useful coatings on the surface of slide fasteners made of aluminum containing as the principal alloying component about 5 per cent of magnesium. In all cases I have observed that the coatings produced are more resistant to chip-'- plng or flaking or wear under normal .abrasive conditions than are the coatings produced in alkali metal carbonate solutions in which the polyvinyl alcohol is not present.

As an example of the abrasion-resistant characteristics of coatings formed in accordance with this invention, I may cite certain abrasion tests made on oxide coated aluminum surfaces produced under comparable conditions. In one instance commercially pure aluminum was treated for 30 minutes, at 87 C. in a solution containing 1 per cent by weight of sodium carbonate, 1 per cent by weight of polyvinyl alcohol, and 0.1 per cent by weight of sodium chromate. The oxide coated surface thus produced was then treated with a hot solution of sulphonated castor oil, and this treatment was followed by a further 15 minute treatment in a 5 per cent solution of sodium ohromate, these after-treatments being treatments applied to oxide coated aluminum surfaces to improve their service characteristics. Another aluminum sample of commercial purity was similarly coated, with the exception that the solution did not contain polyvinyl alcohol. Thereafter it was submitted to the same treatments in sulphonated castor oil and sodium chromate. The two samples were then submitted to an abrasion machine which produced the rubbing type of abrasion, in which a polished steel wheel under pressure is rotated against the revolving surface of the oxide coated sample. The sample prepared in accordance with this invention was, as measured by the results of this abrasion test, more than ten times as resistant to the abrasion as the sample coated in a solution in which polyvinyl alcohol was not present.

The coatings produced by the practice ofthis invention respond to treatment with organic polyvinyl alcohol.

\ 4 dyes, inorganic coloring agents, or similar and other after-treatments normally applied to oxide coatings formed on aluminum.

Having thus described the invention, I claim:

1. The method of chemically forming an oxide coating on aluminum surfaces which comprises treating said surface with an aqueous solution containing, as the active coating formers, 0.5 to 6 per cent by weight of alkali metal carbonate and 0.2 to 3 per cent by weight of polyvinyl alcohol.

2. The method of chemically forming an oxide coating on aluminum surfaces which comprises treatingsaid surface with an aqueous solution containing, as the active coating formers, 0.5 to 3 per cent by weight of sodium carbonate and 0.2 to 3 per cent by weight of polyvinyl alcohol.

3. The method of chemically forming an oxide coating on aluminum surfaces which comprises treating said surface-with an aqueous solution containing, as the active coatingformers, 0.5 to 6 per cent by weight of alkali metal carbonate, 0.05 to 0.5 per cent by weight of alkali metal chromate and 0.2 to 3 per cent by weight of 4. The method of chemically forming an oxide coating on aluminum surfaces which comprises treating said surface with an aqueous solution containing, as the active coating formers, 0.5 to 6 per cent by weight of alkali metal carbonate and 0.2 to, 3 per cent by weight of polyvinyl alcohol, the content of alkali metal carbonate being at least equal to the content of polyvinyl alcohol.

5. The method of chemically forming an oxide coating on aluminum surfaces which comprises treating said surface with an aqueous solution containing, as the active coating formers 0.5 to 6 per cent by weight of alkali metal carbonate and 0.2 to 5 per cent by weight of polyvinyl alcohol.

RALPH B. MASON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,946,150 Tosterud Feb. 6, 1934 2,129,071 Rowell Sept. 6, 1938 2,137,988 Hempel Nov. 22, 1938 2,171,545 Edwards et a1. Sept. 5, 1939 2,279,252 Slunder Apr. 7, 1942 FOREIGN PATENTS Number Country Date 455,412 Great Britain Oct. 20, 1936 

